The present invention relates generally to transistors and more particularly to an improved substrate structure for bipolar and heterojunction bipolar transistors having reduced collector-to-substrate capacitance.
In a conventional bipolar or heterojunction bipolar transistor there is a p.sup.- substrate with a buried layer of n.sup.+ silicon developed thereon. This buried layer is commonly called the subcollector layer. Then a lightly-doped layer of n type silicon is epitaxially grown over the buried layer with this lightly-doped layer being commonly called the collector layer.
To facilitate electrical connection to the collector region of the transistor, the subcollector region extends completely beneath the active region of the transistor and to at least one side thereof to permit electrical contact thereto. This results in a broad p-n junction between the substrate and the subcollector regions with the collector current flowing very close to the p type substrate region. The longer this p-n junction, the greater the substrate-to-collector capacitance of the semiconductor device.
It would be desirable to reduce the substrate-to-collector capacitance in bipolar and heterojunction bipolar transistors to facilitate operation at high frequencies without making it more difficult to establish electrical connection with the subcollector region.